The disposal of municipal solid waste (MSW) presents an increasingly significant and difficult problem for municipalities to solve. As space for landfills diminishes and the permitting for such waste disposal has been eliminated, many schemes have been devised to otherwise utilize the waste as a fuel and generate electricity.
The composition of MSW can vary from locale to locale, but, on the average, has been found typically to be about 50% of combustible materials, such as wood, paper, plastic, food, yard waste and textiles, 5% metals, 10% glass, dirt and stones, and 35% water. The water content varies between 25% and 60% of the waste. To be an efficient fuel, the non-combustible content should be removed, along with the majority of the water. Sorting and magnetic separation can remove a majority of the unwanted components if the waste has been dried, but it is necessary to reduce the water content to 15% or below.
Many schemes have been devised to dewater the MSW, including heating, centrifugal separation, compression, anaerobic digestion etc. However, these schemes have proved to be extremely costly since they require a significant amount of time or energy just to dewater it sufficiently. The heating of wet MSW produces significant, unacceptable odors. As a result of these shortcomings, most MSW is not dried and the waste materials are not separated to improve the fuel value of the waste stream. Instead hundreds of so-called “trash-burning power plants” or incinerators have become economically untenable and hundreds more have not been able to meet the stringent air pollution standards that are prevalent in North America and Europe today.
More recently a process has been introduced which utilizes composting and aerobic techniques to dry the MSW, using closely controlled humidity and temperature. However, this process is so slow that several days are required to reduce the water content sufficiently to produce a high quality biomass fuel.
There is a need for a process and apparatus for disposing of MSW that provides for maximum recycle of useful constituents, removes the inert material and conditions the balance of organic matter (biomass) so that it can be used as a quality fuel to produce electricity or other useful energy in a cost-efficient and pollution-free manner.